Why no depletion mode LDOs?

I seem to recall that there's something about the construction of the usual power FET that makes it only work in enhancement mode -- so a depletion mode power FET would be considerably harder to make. This is probably why they're expensive and rare.

The idea of an LDO with a charge pump so that you can go common-source is interesting -- are there any commercial devices that do this?

Well, if you look at the SOA of this dude here that's mighty impressive:

But ... five bucks :-(

No idea. Dang, should have patented it :-)

With tubes life was quite a bit easier when it came to regulators since they were pretty much all depletion mode devices. But the filament was a pain because sometimes the tube couldn't take much voltage between it and the cathode.

Supertex makes a number of depletion mode FETs available through Mouser for under a dollar in single quantities.

However, big ones like their DN2625 are also around a buck.

Why does using a PMOS pass device give you heartburn? ...Jim Thompson

you can get LDOs with a seperate input pin for a higher voltage bias, so most of it is there

-Lasse

ff

=A0 =A0...Jim Thompson

N.C. FET would allow interesting option with self-controlled regulator with micros.

Because unless I roll my own regulator the things tend to go berserk upon certain load conditions. Very few LDOs are truly stable and those tend to cost a ton.

So if the mfgs can't get most of their designs stable why don't they offer depletion mode?

A lone pin doesn't qualify as "most of it" though :-)

re

h.

uff

is

is

at least it means you have a place to connect the higher supply, if you can find one ;)

-Lasse

re

h.

..

uff

is

is

Take a look at the new thread "What are parts in schematic?". If correct, they are using two dep. N-FET in a CFL. I highly doubt the schematic though. I usually found bipolar 13003/5 inside. You can probably buy the CFL cheaper than the FETs, but remove them before the floating gate oscillator destroy them. That circuit is weir.

Oh. My mistake. I did not see where you stated you needed "big" ones.

I don't need them this big but reasonable, the usual few hundred mA. No problem, I just wondered why none of the big semi mfgs do that.

But I can't possible tell my clients to buy 20 pallets of CFL and take a mallet to the lamps :-)

ent

here

to

uch.

stuff

is is

ce is

Perhaps offer free exchange for used one. Or at least contact the CFL mfr. If they can put them in CFLs, they must have them cheap.

Usually one will not get an answer to such questions, they do not hand out that information. If they haven't sanded it off it may be possible to find out though, by buying and disecting a lamp.

If the pass device is a pfet or pnp transistor, the regulator can be made universally (well, except for pathological loads) stable if the dominant rolloff is a Miller cap, a capacitor from the output back to the gate/base. Adding more external capacitance doesn't add another pole to the loop. Some opamps, like the c-load LM8261, work that way. For a power regulator, the cap might have to be impractically big, but the effect could be simulated.

Somebody should make an LDO that has three external parts: one resistor each to set Vout and Ilim, and an optional cap/RC to control the loop frequency response.

is

LTC's LTC3035 has an internal charge pump they have others with an inductive booster.

kevin

I was told that the reason why that doesn't exist are the Marketing folks. Even one extra pin or one extra external part is going to be dissed in the final design review. IIRC one poster here (Miso?) said something similar as well.

So why not a depletion mode device and be done with the problem?